A planographic printing plate is obtained by imagewise exposing a planographic printing plate precursor to harden exposed portions, dissolving the non-exposed portions of the exposed precursor with a developer, washing the resulting precursor with water, and finisher gum treating the washed precursor. Recently, a method has been studied in which a planographic printing plate precursor is digitally exposed based on image information employing laser ray, and developed with a developer to form an image with high resolution and high sharpness. As one embodiment, there is a system manufacturing a planographic printing plate in which a light sensitive planographic printing plate precursor is scanning exposed employing a light source modulated with an image signal transmitted by communication line or output signal from an electronic plate making system or an image processing system.
However, a conventional planographic printing plate precursor employing a diazo resin has problem in that spectral sensitization to suit a wavelength of laser ray and obtain high sensitivity is difficult.
Recently, a planographic printing plate precursor comprising a photopolymerizable light sensitive layer containing a photopolymerization initiator has been noted as a printing plate precursor for digital exposure employing a laser light, since it is possible to provide a photopolymerizable light sensitive layer highly sensitive to the laser light. A printing plate precursor for CTP (Computer to Plate) system recording digital data employing such a laser light is required to be more highly light-sensitive for the purpose of carrying out recording in a short time.
The planographic printing plate precursor is said to be suitable for forming an image with high sensitivity which comprises a photopolymerizable light sensitive layer containing a photopolymerization initiator, since the photopolymerization initiator imagewise generates radicals on imagewise exposure, and the resulting radicals cause radical chain polymerization as radical initiating species.
In order to attain high sensitivity, various methods have been so far proposed. For example, a method is proposed in Japanese Patent O.P.I. Publication Nos. 63-277653, 63-260909, 1-105238, and 1-203413 in which a tertiary amino group is incorporated in a monomer (an addition polymerizable ethylenically unsaturated monomer). However, methods proposed so far do not provide satisfactory sensitivity or printing durability, and has problem in that when a light sensitive planographic printing plate precursor employing these methods is stored for a long time, its sensitivity is lowered.